1. Field of the Invention
The present invention relates to phase shifters and more particularly to phase shifters with variable time delays.
2. Description of the Prior Art
Phase shifters are generally known in the art. Such phase shifters are used in a relatively wide variety of electronic and microwave applications, such as phased array antenna systems. Examples of such phase shifters are disclosed in commonly owned U.S. Pat. No. 5,606,283, hereby incorporated by reference. Phase shifters can generally be grouped into two categories. One category of phase shifters is based upon materials having a variable permeability. This type of phase shifter typically includes a thin ferrite rod, centered within a rectangular waveguide. A magnetic field applied to the ferrite rod by means of an induction coil wrapped around the waveguide varies the permeability of the ferrite rod, thus controlling the propagation velocity and therefore the phase shift of signals carried by the waveguide. In another type of phase shifter, different signal path lengths are used to control the phase shift of a signal. This type of phase shifter is known to include a bank of switching diodes and various lengths of transmission lines that are switched into or out of the signal path by the diodes to control the propagation delay and therefore the phase shift of the signals carried by the transmission lines.
There is a problem with known phase-shifting devices. In particular, such phase-shifting devices cannot be tuned and thus must be used in applications where the frequency of the input signal is constant. Such devices cannot be used in applications, such as spread spectrum applications, in which the frequency of the input signal varies. Thus there is a need for a phase shifting device which can be programmed in real time to enable the device to be utilized with input signals whose frequency varies.